In a typical optical transmission system, repeater stations may be anywhere from 70 Kilometers (Km) to 200 Km apart. Because optical fibers are presently drawn in individual lengths that are considerably shorter than these distances, the fibers must be spliced together in order to make up the long lengths needed for cabling. These splices must have high strength in order to withstand the high stresses inherent in both cable manufacture and cable installation. In addition, when fiber cables are repaired and must be spliced together, the splices must have high strength to avoid long term static fatigue effects which could cause a complete fracture of the fiber Subsequent repairs could be very expensive, especially if the cable lies on the bottom of the ocean.
At present there are two commonly used techniques for making fusion splices in silica fibers. One technique uses a mixture of two or more combustible gases, such as hydrogen and chlorine or hydrogen and oxygen. The fibers to be fused together are abutted against each other inside the flame produced by the combustion of the gases When the temperature of the fibers is high enough, they fuse together and form a splice. See J. T. Krause et al, "Fibre Splices With `Perfect Fibre` Strengths of 5.5 GPa, v&lt;0.01", Electronic Letters, Vol. 21, No. 12, June 6, 1985, pp. 533-535.
A more commonly used technique employs a small electric arc formed between two metallic electrodes. When the two abutting fibers are placed inside the electric arc, they are heated and fused together. In this regard, see D. L. Bisbee, "Splicing Silica Fibers With an Electric Arc", Applied Optics, Vol. 15, No. 3, March 1976, pp. 796-798.
Fibers made of materials having fusing temperatures of about 700.degree. C. have been fused by heat radiated from an electrically heated nichrome wire. (See, for example, D. L. Bisbee, "Optical Fiber Joining Technique", Bell System Technical Journal, Vol. 50, No. 10, December 1971, pp. 3153-3158). Filament heaters, however, have not been used to fuse silica fibers which fuse at about 2000.degree. C.
Regardless of the splicing method employed, the fiber must be handled with great care in order to avoid reducing its mechanical strength. In particular, fiber strength can be reduced both by mechanical abrasion and by chemical reactants. Mechanical abrasion can cause microscopic scratches on the surface of the fiber resulting in highly localized stresses. When such fibers are stressed due to handling or cabling, the additional stress concentrates in the vicinity of the scratches and can readily lead to a complete fracture of the fiber. Similarly, chemical reaction on the surface of the fiber can develop surface defects which can also cause highly localized stresses. Clearly, both of these causes must be eliminated in order to guarantee long-term strength characteristics.